Off-shore installations, in particular off-shore wind turbines are especially exposed to corrosion due to salt and moisture. Particularly salt is a problem due to its hygroscopic nature and natural ability to transport electrons in corrosion-reactions.
Whilst the outside of the off-shore installation must be designed to bear the brunt of the elements, many of the installations within have trouble coping with the harsh environment in which the off-shore installation must survive. In particularly, the corrosive effects of salt and water combined must be effectively handled in modern off-shore installations to secure uptime and production economy.
Salt and moisture will penetrate into the inside of the off-shore installation in particular through leaks in the outer sheath of the installation, whether such leaks exist by intention of construction or by flaws. Airborne salt from droplets or spray from whitecaps and breaking waves in particular can cause much havoc on sensitive equipment within the off-shore installation.
In an off-shore wind turbine air born salt is known to enter into the inside of both tower and nacelle by leaks, or sometimes “on purpose” when outside air is used to cool inside parts. After some time, the interior structures will be coved by a salt layer that will attract water, and speed up corrosion.
The speed of corrosion is mainly dominated by three factors, 1) relative humidity, 2) salinity, and 3) temperature. Of these relative humidity is by far the most contributing factor.
Numerous studies have tried to describe the correlation between humidity and rate of corrosion in off-shore environments. In general these studies show that the speed of corrosion decreases as the relative humidity is lowered. At a relative humidity of 40-45% there is no corrosion, even though salt is present.
Also ISO9223:2011 points out the relation and based on this classifies corrosion in six classes, going from harmless C1, and up to a very harsh marine environment in C5X.
The classification is important to the cost of manufacturing a wind turbine. The higher classification, the more requirements are put to materials, surface treatments etc. Therefore being able to reduce the classification, in particularly the classification of the interior of the wind turbine, will have significant influence to the price of the wind turbine.
An economical way to protect the wind turbine against corrosion is by lowering the relative humidity by a dehumidifier, e.g. a desiccant dehumidifier. Such systems are well known and have been described in several patent applications such as e.g. WO 2011/091863 or EP 1736665.
Adsorption dehumidifiers, e.g. desiccant dehumidifiers, remove water from a volume of air that passes through it. The heart of this process could be an adsorption rotor made from or coated with a special substance that absorbs the water molecules that make up the moisture in the passing air. As the rotor absorbs water it becomes necessary to remove the absorbed water in order to regain the adsorption capability of the rotor. In order to remove the humidity in the rotor, the rotor is rotated over to a regeneration zone, where it is dried with heated air. The warm, humid regeneration air is led out, and the rotor is once again ready to absorb water molecules. This operation can be continuous or stepwise.
FIG. 1 describes a dehumidification system commonly installed in off-shore wind turbines. In the depicted setup, there are two separate air flows, process air (air to be dried) and regeneration air (air for removing moisture from rotor). This is normally a desired setup; as such a setup is pressure neutral to the room and would also be desired for off-shore wind turbine applications, only it introduces a problem as the system requires regeneration air to be taken from outside. The outside air is salty with droplets, aerosols, sprays etc. containing salt. Normally, it is very difficult to adequately arrest salt in a normal filter, as the filter will get wet, and the saline water will pass. This reduces the efficiency of the filter with a high risk of salt particles passing it. Further, if the saltwater is arrested, the salt will become sticky, and filter is in risk of becoming blocked.
FIG. 2 describes a dehumidification system commonly installed in off-shore wind turbines and directed to producing an over-pressure within the turbine tower. This has amongst others the benefits of reducing the inflow of moist and corrosive air through leaks etc. in the tower and turbine nacelle; to cool down inner parts (gears, generators, switches, etc.).
In FIG. 2, outside air is dehumidified and blown into the wind turbine. This will create an over-pressure. In the drawing regeneration air is taken from outside, but could also be taken from inside and like the normal pressure setup, also the displayed over-pressure setup will have problems with inefficient filters and filters clogging, due to salt.
EP 1736665 discloses a method of drying an inner space—sealed off or at least substantially sealed off from air exchange—of a wind energy plant with a dehumidifying device connected to the inner space by way of a moist air inlet and a dry air inlet and arranged between them, in that by drawing air out of the inner space through the moist air inlet a region of under-pressure is produced in the inner space, and by expelling dried air through the dry air outlet into the inner space a region of over-pressure is produced in the inner space, and between the under-pressure region and the over-pressure region a drying airflow is produced in the inner space, and the moist air inlet and the dry air outlet are kept at least a distance from each other by an air duct.
WO 2011/091863 discloses a system for the treatment of indoor air, in particular for the treatment of indoor air in the interior space of a tower of a wind energy plant, having an adsorptions system wherein from at least one dehumidification sector of an adsorption rotor is delivered dry or dried feed air to said interior space, and at least one regeneration sector of said adsorption rotor by way of an air-inlet from the exterior of said space is supplied with humid and salty exterior air as regeneration air, and wherein the moisture absorbed in the dehumidification sector of said adsorption rotor is released via an air-outlet to the exterior of said space characterized in that a heat exchanger for the regeneration air with air conduits sealed hermetically from one another, a first air conduit connected at the entry side with said air-inlet and at the exit side with said regeneration sector of said adsorption rotor, and a second air conduit connected at the entry side with said exit of said dehumidification sector and/or said exit of said regeneration sector of said adsorption rotor and with the exit side of said air-outlet and/or with said space.
WO 2011/091863 additionally foresees the use of a salt filter located downstream from the heat exchanger in direction of the airflow before the airflow reaches the adsorption rotor.
However, the problem with outside salt, i.e. salt that is entering the dehumidifying system from the exterior of the wind turbine; is that it is in a moist state and thus is hard to arrest. This leads to a risk of salt passing the filter, a reduced lifetime of the salt filters installed, and to unwanted repairs and downtime of such dehumidification systems as described in the known art.
It is thus an object of the present invention to improve upon the known art by making available a system for and a method of dehumidifying the interior of an off-shore installation, in particular an off-shore wind turbine, wherein the efficiency and the lifetime of salt filters installed as part of such a dehumidification system are significantly improved and thereby costs for maintenance and downtime for repairs for such an off-shore installation reduced.